Antibodies are a class of proteins called immunoglobulins. They consist of 4 protein chains linked by disulphide bonds (figure 1).

Figure 1 - Structure of an antibody

There are two pairs of heavy chains (H) and two light chains (L).

Antibodies have two structurally distinct domains. The antigen binding domain of the molecule is highly variable and therefore referred to as the variable (V) domain. The remainder of the molecule is relatively constant and therefore referred to as the constant (C) regions (Figure 1).

Within the variable domains, three complementarity determining regions (CDR) on both the H and L chains are the regions with most hypervariability. The CDRs confer the specificity (recognition) and most of the affinity (binding) of the antibodies for their target antigens.

The constant region (C) of both the H and L chains can be of several distinct classes, also referred to as isotypes. The L chain can be either kappa or lambda, while the H chain can be of five different classes IgM, IgD, IgG 1-4, IgA and IgE.